Horrocks S J, Ritchie G A D, Sharples T R
Physical and Theoretical Chemistry Laboratory, The University of Oxford, South Parks Road, Oxford, OX1 3QZ, UK.
J Chem Phys. 2007 Jan 28;126(4):044308. doi: 10.1063/1.2429656.
The technique of resonance enhanced multiphoton ionization (REMPI) has been used in conjunction with time-of-flight mass spectrometry (TOFMS), to investigate the dynamics of ozone photolysis in the long wavelength region of the Hartley band (301-311 nm). Specifically, both the translational anisotropy and the rotational angular momentum orientation of the O(2) (a (1)Delta(g); nu=0, J=16-20) fragments have been measured as a function of photolysis wavelength. Within this region, the thermodynamic thresholds for the formation of these products in combination with O ((1)D(2)) are approached and passed, and consequently these studies have allowed an investigation into the effects on the dynamics of slowing fragment recoil velocities and the increasing importance of vibrationally mediated photolysis. The determined beta parameters for all the J states probed follow a similar trend, decreasing from a value typical for the initial (1)B(2)<--(1)A(1) excitation responsible for the Hartley band [for example, beta=1.40+/-0.12 for the O(2) (a (1)Delta(g); J=18) fragment], to a much lower value beyond the thermodynamic threshold for the fragment's production (for example, beta=0.63+/-0.19 for the J=18 fragment following photolysis at 311 nm). This trend, similar to that observed when probing the atomic fragment in a previous set of experiments, [Horrocks et al., J. Chem. Phys. 125, 133313 (2006); Denzer et al., Phys. Chem. Chem. Phys. 16, 1954 (2006)] is consistent with the photodissociation of vibrationally excited ozone molecules beyond the threshold wavelengths and we estimate approximately 1/3 of this to be from excitation in the nu(3) asymmetric stretching mode. These observations are substantiated by the values of the beta(0) (2)(2,1) orientation moment measured, which for photolysis at 301 nm are negative, indicating that a bond opening mechanism provides the key torque for the departing O(2) fragment. The orientation moment becomes positive again for photolysis beyond threshold, however, as the increasing impulsive dissociation again begins to dominate the nature of the rotation of the departing molecular fragment. In addition, a (2+2) REMPI scheme has been utilized to probe the O(2) (a (1)Delta(g)) "low" J fragments, where the majority of the population resides following photolysis within this region. The REMPI-TOFMS technique has been used to confirm the rotational character of a spectral feature through examination of the signal line shapes obtained using different experimental geometries. The dynamical information subsequently obtained, probing the "low" J O(2) (a (1)Delta(g)) fragments on these rotational transitions, has unified previous translational anisotropy results obtained by detecting the O ((1)D(2)) atomic fragment with data for the O(2) (a (1)Delta(g); J=16-20) fragments.
共振增强多光子电离（REMPI）技术已与飞行时间质谱（TOFMS）结合使用，以研究哈特利带长波长区域（301 - 311 nm）中臭氧光解的动力学。具体而言，已测量了O(2)（a (1)Delta(g); ν = 0, J = 16 - 20）碎片的平动各向异性和旋转角动量取向随光解波长的变化。在该区域内，形成这些产物与O ((1)D(2)) 相结合的热力学阈值被接近并超过，因此这些研究得以探究对动力学的影响，即减缓碎片反冲速度以及振动介导的光解的重要性增加。所探测的所有J态的确定β参数遵循相似趋势，从负责哈特利带的初始(1)B(2)<--(1)A(1)激发的典型值下降 [例如，对于O(2)（a (1)Delta(g); J = 18）碎片，β = 1.40±0.12]，到超过碎片产生的热力学阈值时的低得多的值（例如，在311 nm光解后J = 18碎片的β = 0.63±0.19）。这种趋势与在先前一组实验中探测原子碎片时观察到的趋势相似，[霍罗克斯等人，《化学物理杂志》125, 133313 (2006); 登泽尔等人，《物理化学化学物理》16, 1954 (2006)] 与超过阈值波长的振动激发臭氧分子的光解离一致，并且我们估计其中约1/3来自ν(3)非对称拉伸模式的激发。所测量的β(0) (2)(2,1)取向矩的值证实了这些观察结果，对于301 nm的光解，其为负，表明键打开机制为离去的O(2)碎片提供了关键扭矩。然而对于超过阈值的光解，取向矩再次变为正，因为增加的脉冲解离再次开始主导离去分子碎片的旋转性质。此外，已利用(2 + 2) REMPI方案探测O(2)（a (1)Delta(g)）“低”J碎片，在该区域光解后，大多数粒子驻留在这些碎片中。REMPI - TOFMS技术已用于通过检查使用不同实验几何结构获得的信号线形状来确认光谱特征的旋转特性。随后获得的动力学信息，探测这些旋转跃迁上的“低”J O(2)（a (1)Delta(g)）碎片，已将先前通过检测O ((1)D(2))原子碎片获得的平动各向异性结果与O(2)（a (1)Delta(g); J = 16 - 20）碎片的数据统一起来。
